Ryuichi Arafune

Structure of Silicene Grown on Ag(111)

The structure of silicene, the two-dimensional honeycomb sheet of Si, grown on Ag(111) was investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) combined with density functional theory (DFT) calculation. Two atomic arrangements of honeycomb configuration were found by STM, which are confirmed by LEED and DFT calculations; one is 4×4 and the other is √13×√13 R13.9°. In the 4×4 structure, the honeycomb lattice remains with six atoms displaced vertically, whereas the √13×√13 R13.9° takes the regularly buckled honeycomb geometry.

Determination of molecular orientation of very thin rubbed and unrubbed polyimide films

To determine the molecular orientation of rubbed and unrubbed poly[4,4′‐oxydiphenylene‐ pyromellitimide] (PMDA‐ODA) films, we have measured the infrared (IR) absorption spectra of PMDA‐ODA films on Si substrates as a function of the incident angle. The molecular orientation was determined by fitting the incident angle dependence and the IR dichroic ratio with the theoretical curves. The IR absorption was calculated by a transfer matrix method to take account of the multiple reflection and refraction at the interfaces. The vibrational response of the polyimide film in the IR region was represented by a Lorentz oscillator model. We found that the molecules in a spin‐coated polyimide film are oriented parallel to the substrate surface with a standard deviation of 6.5°. For a rubbed film, the polymer chains are oriented along the rubbing direction and are tilted up on average by 8.5° from the surface plane. We observed a blueshift of the C=O asymmetric stretching band at oblique incidence. The blueshift is ex...

CORRELATION BETWEEN THE PRETILT ANGLE OF LIQUID CRYSTAL AND THE INCLINATION ANGLE OF THE POLYIMIDE BACKBONE STRUCTURE

We have determined the inclination angle of the backbone structure of polyimide with alkyl side-chains in rubbed films. Four different polyimides with the same backbone structure but different lengths of alkyl side-chains were used in this study. The inclination angle of the backbone structure increases with the number of carbon atoms in the alkyl side-chains. We found a linear relation between the inclination angle of the backbone structure and the pretilt angle of liquid crystal (LC) that is in contact with the polyimide films. We conclude that the inclination of the polyimide backbone structure determines the pretilt angle of LC.

Surface anisotropy of polyimide film irradiated with linearly polarized ultraviolet light

Using polarized infrared (IR) absorption, we have investigated the surface anisotropy of a poly [4, 4′-oxydiphenylene-pyromellitimide] (PMDA-ODA) film that arises from anisotropic decomposition of the polyimide chain during irradiation with linearly polarized ultraviolet (LPUV) light. To monitor the surface anisotropy, we designed the sample structure so that the polyimide films decomposed uniformly over the entire film thickness. The surface anisotropy has a maximum at an irradiation energy of 105 J/cm2. For PMDA-ODA, the maximum surface anisotropy is significantly smaller than the surface anisotropy generated by rubbing. By analyzing the irradiation energy dependence of an IR absorption band, we found that the decomposition rate of the polyimide chain oriented parallel to the polarization direction of the LPUV light is greater only by ∼23% than that oriented perpendicular to it. This is the reason for the small surface anisotropy induced by the LPUV light irradiation.

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

Abstract The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.

Relation between the molecular orientations of a very thin liquid crystal layer and an underlying rubbed polyimide film

By polarized infrared absorption spectroscopy, we have determined the relation between the molecular orientation of a rubbed polyimide film and that of a very thin liquid crystal (LC) layer (approximately a monolayer of 8CB molecules) in contact with the polyimide film. The molecular orientation of the rubbed polyimide film was determined by fitting theoretically the incident angle dependence of the infrared absorption for molecular vibrations polarized parallel to the length of the polymer. Approximately a monolayer of 8CB molecules was deposited on a rubbed polyimide film with previously measured molecular orientation. The molecular orientation of this LC layer was also determined by polarized infrared absorption. We found that the LC molecules and the polyimide chains are oriented, on average, along the rubbing direction and tilted up from the surface, and that the molecular order of the LC layer is higher than that of the underlying rubbed polyimide film.

Electronic decoupling by h-BN layer between silicene and Cu(111): A DFT-based analysis

Geometric and electronic structures of silicene on Cu(111) covered with a monolayer of hexagonal boron nitride (h-BN) were investigated by ab initio density functional theory calculations. We found that a silicene with a regularly buckled configuration is stabilized on h-BN layer stacking commensurately to the Cu(111) substrate. The electronic band structure projected to Si 3pz orbital clearly shows a band crossing similar to a Dirac cone emerging in the band structure of freestanding buckled silicene. This is in contrast to the silicene on Cu(111), in which the Dirac fermion features disappear entirely due to the strong interactions at the interface. These examples demonstrate that the h-BN monolayer effectively prevents silicene from interacting with the underlying Cu(111) substrate and that the h-BN monolayer on Cu(111) is a promising candidate for use as a substrate on which to realize silicene hosting the Dirac fermion features.

Pretilt angles of liquid crystals in contact with rubbed polyimide films with different chain inclinations

Abstract We have measured the pretilt angles of liquid crystals in contact with rubbed polyimide films (poly[4,4′-oxydiphenylene-pyromellitimide]; PMDA-ODA) that have different inclination angles of the polyimide chain. To produce polyimide films with different chain inclination angles, one film was rubbed in one direction only and the other film was rubbed twice in opposite directions. The average orientation of the polyimide chain was determined by polarized infrared absorption. The pretilt angle of the liquid crystal was measured by an improved crystal rotation method. We found that the pretilt angle increases with the increase of the inclination angle of the polyimide chain. This result indicates that the pretilt angle is mainly determined by the inclination angle of the polyimide chain in the film in contact.

Electronic Structure of the 4 × 4 Silicene Monolayer On Semi-Infinite Ag(111)

The electronic structure of the 4 × 4 silicene monolayer on a semi-infinite Ag(111) substrate is calculated within density functional theory by using the embedded Green’s function technique. The present calculation confirms the conclusion of previous studies that the two-dimensional (2D) Dirac bands do not exist on this surface as a result of the symmetry breaking and strong orbital hybridizations between the Si π and Ag sp states. In addition, by making use of the advantage of the semi-infinite calculation in which the energy continuum of the bulk Ag bands is fully reproduced, we investigate details of the silicene-induced electronic states, including not only their energy dispersion with 2D wave vector but also their spectral shape as a function of energy at each .

One-dimensional edge state of Bi thin film grown on Si(111)

The geometric and electronic structures of the Bi thin film grown on Si(111) were investigated by using scanning tunneling microscopy and spectroscopy. We have found two types of edges, one of which hosts an electronic state localized one-dimensionally. We also revealed the energy dispersion of the localized edge state from the evolution of quasiparticle interference patterns as a function of energy. These spectroscopic findings well reproduce those acquired for the cleaved surface of the bulk Bi crystal [I. K. Drozdov et al., Nat. Phys. 10, 664 (2014)]. The present results indicate that the deposited Bi film provides a tractable stage for further scrutiny of the one-dimensional edge state.